1. Field of the Invention
This invention relates to a water-absorbent agent, a method for the production thereof, and a water-absorbent composition. More particularly, it relates to a water-absorbent agent manifesting as high an absorption capacity even under load as without load, excelling in safety as evinced by the absence from the resin surface of a residue of the cross-linking agent of high reactivity, and befitting sanitary materials and to a method for the production thereof. It also relates to a novel water-absorbent composition which exhibits an excellent water-absorbent property to such body liquid as urine, blood, and secretions, enjoys unusual gel stability, and also excels in the flowability after absorbing moisture, the ability to resist static charge, and the rustproofing effect.
2. Description of the Prior Art
In recent years, the water-absorbent resins have been in extensive use as one of the component materials of such sanitary materials as disposable diapers and sanitary napkins for the purpose of absorbing body liquid.
The water-absorbent resins of this class heretofore known to the art include partially neutralized cross-linked polyacrylic acids (JP-A-55-84,304, JP-A-55-108,407, JP-A-55-133,413, U.S. Pat. No. 4,654,039, and U.S. Pat. No. 4,286,082), hydrolyzed starch-acrylonitrile graft polymers (JP-A-46-43,995 and U.S. Pat. No. 3,661,815), neutralized starch-acrylic acid graft polymers (JP-A-51-125,468 and U.S. Pat. No. 4,076,663), saponified vinyl acetate-acrylic ester copolymers (JP-A-52-14,689 and U.S. Pat. No. 4,124,748), hydrolyzed acrylonitrile copolymers or acrylamide copolymers (JP-A-53-15,959, U.S. Pat. No. 3,935,099 and U.S. Pat. No. 3,959,569), cross-linked derivatives thereof, cross-linked carboxymethyl cellulose (U.S. Pat. No. 4650716 and U.S. Pat. No. 4689408, and cross-linked polymer of cationic monomers (JP-A-58-154,709, JP-A-58-154,710, U.S. Pat. No. 4,906,717, U.S. Pat. No. 5,075,399, and EPO304143), cross-linked isobutylene-maleic anhydrous copolymers (U.S. Pat. No. 4,389,513), and cross-linked copolymers of 2-acrylamide-2-methylpropanesulfonic acid with acrylic acid (EPO68189), for example.
The characteristic qualities which the water-absorbent resins are desired to possess include high water-absorption capacity and fine speed of absorption to be manifested on contract with aqueous liquids, liquid permeability, high strength exhibited by the gel swelled with liquid, and ability to aspirate water from the substrate impregnated with aqueous liquid, for example. These qualities are not necessarily correlated positively to one another. For example, such physical properties as the liquid permeability, the gel strength, and the absorption speed are lowered in proportion as the water-absorption capacity is heightened.
As a measure to improve the various water-absorption properties of the water-absorbent resins in finely balanced levels, the technique of cross-linking the surface regions of water-absorbent resin particles has been known. Various methods have been heretofore proposed concerning the technique.
For example, methods using polyhydric alcohols (JP-A-58-180,233, JP-A-61-16,903, U.S. Pat. No. 4,734,478, and U.S. Pat. No. 5,164,459), a method using polyglycidyl compounds, polyadiridine compounds, polyamine compounds, and polyisocyanate compounds (JP-A-59-189,103), methods using glyoxal (JP-A-52-117,393 and U.S. Pat. No. 4,051,086), methods using polyvalent metals (U.S. Pat. No. 4,043,952, JP-A-51-136,588, JP-A-61-257,235, and JP-A-62-7,745), methods using a silane coupling agent (U.S. Pat. No. 4755560, JP-A-61-211,305, JP-A-61-252,212, and JP-A-61-264,006), a method using a monoepoxy compound (JP-A-61-098,121), a method using an epoxy group-containing polymeric compound (U.S. Pat. No. 4,758,617), a method using an epoxy compound and a hydroxy compound (JP-A-02-132,103), and a method using an alkylene carbonate (DE-4020780) severally as a cross-linking agent have been known. Besides, methods requiring the presence of an inactive inorganic powder (JP-A-60-163,956, JP-A-60-255,814, and U.S. Pat. No. 4,587,308), a method requiring the presence of a dihydric alcohol (JP-A-01-292,004), a method requiring the presence of water and an ether compound (JP-A-02-153,903), and a method the presence of the alkylene oxide adduct of a monohydric alcohol, an organic acid salt, lactam, etc. (EP-0555692) in the process of a cross-linking reaction have been known.
These methods indeed improve the balances of the various physical properties of the absorbent resins. The degrees of these improvements, however, hardly deserve to be rated fully satisfactory. Thus, the water-absorbent resins have room for further improvement in qualities. Particularly, in recent years, the desirability of perfecting a water-absorbent resin which is capable of exhibiting outstanding water-absorption properties under load, especially water-absorption capacity under load while keeping at a high level the water-absorption capacity without load, i.e. one of the basic physical properties of the conventional water-absorbent resins, has been finding popular recognition. Of course, the water-absorption capacity without load and the water-absorption capacity under load are generally in a contradicting relation. With all the methods heretofore known to the art, the needs of further technical improvements have not been fully satisfied.
Besides, the water-absorbent resins have the problem of requiring use of a cross-linking agent in cross-linking the surface region of water-absorbent resin particles and inevitably suffering this cross-linking agent to persist as a residue on the surface of resin particles. This problem is absent when the cross-linking agent so used has low reactivity and high safety as in the case of a polyhydric alcohol. When the cross-linking agent has high reactivity as in the case of an epoxy compound, though the cross linking in the surface region proceeds quickly and, the fact remains that the cross-linking agent itself has the nature of irritating the skin and, when suffered to persist on the surface of resin, jeopardizing the safety of sanitary materials using the cross-linked absorbent resin. This gives rise to a new problem from the viewpoint of safety.
The method which, for the purpose of decreasing the amount of the fraction of a cross-linking agent being used for cross-linking the surface region which persists as a residue in the resin, comprises initiating the cross-linking reaction of the surface region of resin at a specific water content and adding a specific amount of water to the cross-linking reaction system while the reaction is in progress has been known (JP-A-03-295,705). This method, however, not only entails a complicated process but also fails to decrease the amount of a residue of the cross-linking agent sufficiently.
The methods described above fulfill, though not thoroughly, the improvement of the balances of various physical properties of water-absorbent resins and the enhancement of the absorption capacity of the water-absorbent resins under load. They at times prove undesirable from the viewpoint of safety because the cross-linking agent used for cross-linking the surface region of the water-absorbent resins particles possibly leaves behind a persistent residue in the polymer.
They have further problems of low flowability (blocking) with the high hygroscopicity of the produced water-absorbent resins itself which renders difficult the work of fabricating the water-absorbent resins under the conditions of high temperature and high humidity, the susceptibility of the water-absorbent resins to static charging which degrades the ease of handling, and the ability of the water-absorbent resins to gather rust on metal which induces corrosion of the devices to be used for the work of fabrication. Further, when the liquid to be absorbed is blood, the absorption capacity of the water-absorbent resins is degraded because the blood components of the blood being absorbed by the water-absorbent resins envelope the individual water-absorbent resins particles and eventually obstructing them from continuing their function as a water-absorbent. The water-absorbent resins heretofore known have not been always given fully satisfactory results particularly in such applications as sanitary napkins, for example.
Then a method which attains the purpose of improving the water-absorbing power of water-absorbent resins relative to blood by adding such compounds as common salt and polyether to the water-absorbent resins has been proposed (JP-A-58-501,107, JP-A-55-50,357, JP-A-54-70,694, U.S. Pat. No. Re 33839, EP-A-0009977, U.S. Pat. No. 4,190,563, etc.). Indeed, this method is effective to some extent in preventing the individual water-absorbent resins particles from conglomeration, it actually cannot bring about the expected effect because the added compound either degrades the water-absorbent resins' own absorption capacity or fails to increase the suction power to blood.
An object of this invention, therefore, is to provide an absorbent agent manifesting as high an absorption capacity even under load as without load, excelling in safety as evinced by the absence from the resin surface of a residue of the cross-linking agent of high reactivity, and befitting sanitary materials and to a method for the production thereof.
Another object of this invention is to provide a novel water-absorbent composition, wherein a water-absorption capacity, especially an absorption property to blood is sufficient and workability and safety are satisfied, a method for the production thereof, and a water-absorbent structure and a water-absorbent article both containing the water-absorbent composition.
The water-absorbent composition of this invention is adapted for disposable diapers, sanitary napkins, jigs for use on patients of incontinence, such water-absorbent articles as injury protecting materials and injury curing materials which serve the purpose of absorbing body liquid, or biotechnology applications.